In essence, it is well-known in the technology which is concerned with the extrusion of molten thermoplastic tubular film to provide an apparatus and method which will control the internal air pressure in the film bubble upon the extrusion thereof through an annular die orifice, and prior to effectuating the collapsing of the tube between suitable pinch rolls or the like which draw the expanded bubble in a longitudinal or machine direction, which rolls also serves to flatten the formed tube into a double thickness sheeting adapted to be wound into a cylindrical roll for storage and subsequent use.
Basically, the gaseous medium which is employed to inflate the tubular film bubble as the latter is being extruded from an extrusion die orifice is forced into the interior of the tubular film through a suitable inlet in the extrusion die under a generally constant pressure which is adequate to produce a desired radial expansion of the extruded tube, and resultantly a molecular orientation of the successive portions of the tubular film subsequent to exiting from the extrusion die and while being externally cooled to an orientation temperature. Generally, in various physical applications the external cooling of the film bubble may be implemented through impinging cooling air supplied by one or more annular cooling air rings encompassing the tube or pursuant to an alternative arrangement, as in the present instance, by passing the inflated film tube or bubble upwardly through a surrounding water bath cooling apparatus which produces the rapid quenching of the extruded tubular thermoplastic film which is in a molten state so as to enhance the speed of cooling thereof and increase the strength of the film material.
Thus, for instance, a water bath film cooling apparatus and method which may be readily employed in connection with the present invention for controlling the bubble pressure is disclosed in applicant's U.S. Pat. No. 4,927,574, which is commonly assigned to the assignee of the present application, and the disclosure of which is incorporated herein by reference. In that disclosure, a water column communicating with and conforming to the vertical height of a cooling water bath surrounding the film tube is employed to assert in maintaining a predetermined optimized pressure differential between the outside of the film tube or bubble being extruded over the height of the liquid cooling medium or water bath and the interior of the film tube so as to provide for an improved cooling operation and enabling control over the size o diameter of the film bubble.
In accordance with the disclosure of Herrington, et al. U.S. Pat. No. 4,069,292, also assigned to the common assignee of the present application, and the disclosure of which is incorporated herein by reference, control is exercised over the pressure which reigns internally of a molten tubular film being extruded from an extrusion apparatus to form a film bubble, in that a Photohelic pressure controller may be employed to continually measure the internal pressure of the thermoplastic tube which is being extruded, and if the pressure deviates beyond a range set by upper and lower set points on the controller, to either slowly or rapidly introduce or withdraw air from the interior of the film bubble being extruded so as to exercise a regulating effect over the dimensions or diameter of the bubble. In this patent, the cooling of the outside surface of the film bubble is effectuated through the intermediary of impinging an air flow against the surface by positioning an air ring downstream of the extrusion die orifice.
However, during an upward extrusion of a tubular film which is being conducted through a cooling water bath rather than being cooled by a flow of air, the internal pressure of the film bubble must be controlled to some specified amount which is greater than the external pressure exerted at the extrusion die by the head of water from the water bath. Thus, as the depth of the water is increased during the thread-up of the system, the controlled bubble pressure must increase correspondingly so as to always remain higher by the same differential amount than that of the water depth presented by the cooling water bath. Consequently, during thread-up, the bubble pressure prevailing within the tubular film must increase from, for instance, from 4" to 124", while the depth of the water bath rises from 0 to 120". In this connection, it is extremely difficult to maintain this difference by a direct control of bubble pressure, inasmuch as the set point of the employed pressure controller must be continually readjusted to conform with the changing conditions encountered during this time period.
Although various methods and apparatus have been employed in the thermoplastic tubular film extrusion technology to control film bubble pressure, none of these are in essence applicable to utilization of controlling bubble pressure, particularly during the thread-up of the extruding system, in which the tubular film is conducted upwardly through a cooling water bath imparting an external pressure against the surface of the tubular film bubble over a varying height.
U.S. Pat. No. 4,069,292, a co-inventor of which is the inventor of the present invention, primarily employs a Photohelic pressure controller in conjunction with regulating the internal pressure of a film bubble, and in which the external cooling is effected through the intermediary of an air flow impinging against the outer surface of the film bubble. Consequently, the structure and function of the Photohelic pressure controller disclosed in this particular patent, and which acts only with gaseous media, such as air, is not adapted for use with an extrusion system for tubular films employing a cooling water bath, and particularly in attempting to control the internal bubble pressure during the thread-up of the system.
Co, U.S. Pat. No. 4,699,580, discloses a system for controlling the diameter of a blown film tube. However, there is no disclosure of methods or apparatus for controlling the pressure differential between the interior of a film tube and an external cooling water bath surrounding the tube analogous to that contemplated by the present invention.
Cluett, et al., U.S. Pat. No. 4,377,540, disclose a system for controlling the diameter of an extruded film tube or bubble which is cooled internally, and which incorporates electronic sensor devices for regulating the flow of air. This disclosure does not teach controlling the pressure prevailing within an extruded tubular film in conformance with the external pressure of a cooling water bath through which the tube is conducted upwardly.
Mulcahy, U.S. Pat. No. 4,243,363, discloses a physical sensing system for contacting an extruded film tube so as to regulate the size thereof by varying the internal pressure. Again, there is no correlation with that particular patent and the present invention since the patent does not disclose a water bath surrounding the film tube.
Schott, Jr., U.S. Pat. No. 4,402,656, discloses a system for measuring the diameter of an extruded film tube and upon deviation in the size thereof, through photo-optical sensing devices enabling controlling the size of the film tube by regulating an internal airflow.
Finally, Sweeney, et al., U.S. Pat. No. 4,355,966, disclose the automatic control of film bubble size for a blown tubular film by measuring the diameter thereof and, by means of an electronic system, adjusting the air flow being supplied to the interior of the tube which is being extruded.